Wireless communication is known in the art. A conventional wireless transmitter produces an electromagnetic signal, which is transmitted over a medium. This medium is often non-perfect, being rural or urban, and induces reflections and other disturbances, which cause the signal to fade. This phenomena is called multi-path.
Cellular mobile communication attempts to provide mobility, multi-user capacity (many independent users access the system), coverage (service is offered over a large contiguous area) and grade and quality of service.
Cellular communications are generally limited by local codes to a range of frequencies. A widely used technique of cellular communications employs spatial isolation in order to be able to reuse the same frequencies beyond a given range called a guard zone. The communications of each user is maintained with a base station, whose antenna is elevated above the scenery in order to achieve a well defined and controlled coverage area. Sectorization is achieved by directive antennas that illuminate only one sector, thereby reducing interference, enhancing performance and reducing a pattern of frequency reuse.
The number of concurrent calls communicating with Each sector of cellular communications is limited by the frequency band assigned to the service, by the technology used and by the frequency reuse pattern. The number of calls per unit area, also called area capacity, can be increased by reducing the cell size. Small cells that are positioned below roof tops in urban areas are called microcells. These use lower and smaller antennas. The cell hardware is more compact, and in some cases has less circuits. Another technique for microcells involves the antenna and RF circuitry only, remote from the cell equipment and connected via RF, fiber or microwave link, to the cell. Such an arrangement is especially attractive for operators in possession of RF or fiber trunking, like CATV companies.
Electromagnetic radiation is polarized and allows for two orthogonal polarization states It will be appreciated by those skilled in the art that an antenna can only transmit in a single polarization state.
The propagation of the signals through an inhomogeneous medium and through scattering may transfer part of the signal to the orthogonal polarization. This is the case for terrestrial communications, for example, and in particular in urban areas, where the signals encounter mutipaths from objects on the way.
The transfer of polarization has been found to be typically −10 dB in rural areas, −7.8 dB in urban areas and as high as −4 dB indoors [See for example Jorn Toftgard and Patrick C. F. Eggers: Experimental Characterization of the Polarization State Dynamics of Personal Communication Radio Channels, Proc. IEEE VTC'93, pp. 65–69].
The orthogonal polarization components have been found to have an independent fading pattern, with correlation lower than 0.6 and similar fading statistics.
Diversity techniques are used in wireless communications to mitigate the degradation due to signal fading. In space diversity, the antennas for space diversity are spaced apart enough for the fading of the waves arriving to each to be time-independent from those arriving at the other. The spacing required between the antennas is inversely proportional to the angle of intercept of the arriving waves. Accordingly, often the distance between such two antennas is chosen to be considerably large.
A repeater in the cellular system is a device that receives the transmission from the Base Station (the donor side) and retransmits it to the subscribers (the distribution side) with proper amplification. Simultaneously it receives the signals from the subscribers and retransmits it, with proper amplification, to the Base Station. Repeaters are used mainly for the following applications:                Providing RF coverage in areas where the signal received from the Base Station is too week (“Radio Holes”)        Extending the cell coverage, e.g. along highways        Extending the coverage into tunnels, buildings or other structures.        
A repeater has to be transparent—the grade of service should not be degraded by the introduction of the repeater in the link. The repeater has to cover the frequency range allocated to the distribution area to be covered. Preferably it is the whole frequency range of the Base Station. The repeater has to have alarms, status reporting and controls, to be controlled from the Base Station, either via land lines or via transmissions. The lack of diversity in the repeaters hampers their performance.